talk 270717.ppt [kompatibilitätsmodus]
TRANSCRIPT
27. July 2017
From "plug & play" towards a better understanding
of scanning probe microscopy imaging
University of Bonn
Elisabeth Soergel
… something else …
What to do?
4
Jan Brueghel the Elder (1568 - 1625)Flowers in a Wooden Vessel, 1603
Kun
sthi
stor
isch
es M
useu
m W
ien.
Scanning Probe Microscopya bouquet of possibilitiesfor investigating samples
Lithium Niobate a rich playground
for fascinating science
7
Scanning Force Microscopy (SFM) Cantilever movement readout
Vertical read-out channel: Deflection & Buckling
vertical
vertical
lateral
Torsion Bucking:Rotation of the sample by 90°
Lateral read-out channel: Torsion
8
Scanning Force Microscopy (SFM) Calibration of in-plane displacements (torsion & buckling)
• Use two identical piezoplates
• Use calibrated height scanner
to calibrate the piezoplate
• Find appropriate
amplitude & frequency (2Vpp / 10kHz)
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Scanning Force Microscopy (SFM) Crosstalk between the signals?
"phantom"lateral signal< 1%
0 50 100 150
-1.0
1.0
0.0
Am
plitu
de [
V]
Time [µs]
no compensation
vertical
lateral
0.0
0 50 100 150Time [µs]
with compensation
-0.1
0.1zoom
Mechanical (sample topography)Slow scanning
Electronical (manufacturer's fault)Better shielding
Misalignment Crosstalk-compensator
Excite the cantilever at its resonance frequency
RSI 78, 016101 2007
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Scanning Force Microscopy (SFM) Electrostatic Force Microscopy (EFM)
SFM in non-contact mode: tip-sample distance z few nm
Lateral resolution ∝ r and z
Frequency: few 10 kHzAmplitude: few V
Sensitivity: Single(!) charge detection (fN)
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Scanning Force Microscopy (SFM)
Voltage VACapplied to the tip
Force Facting on the tip
Electrostatic Force Microscopy (EFM)
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Scanning Force Microscopy (SFM) Electrostatic Force Microscopy (EFM)
Photoconductivityof Bi12SiO20
Corona discharges(± 10 V, 10 ms)on Bi12SiO20
500 nm tickdielectric layer
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Surface Charge DensityDetermination of the Surface Charge Density (SCM)
Probe can no longer be approximated by a sphere
Cone, cantilever, … have to be taken into account
= a sample with known surface charge density impossible
= a sample with known, controllable change of the surface charge density pyroelectric crystal
Need of a calibration sample
With a known pyroelectric coefficient dPs /dTa controlled change ΔT leads to a known ΔQsurface
14
Surface Charge DensityCalibration & Results for the SCD on LiNbO3
LiNbO3: dPs/dT = 80 x 106 C/Km²
LiTaO3: dPs/dT = 190 x 106 C/Km²
stabilized Peltier [ΔT in min]heating resistor [ΔT in s]
LiNbO3
Temp. steps ΔT: 0.5 to 2.5 K
[ΔT in min] or [ΔT in s] yielded the same results.
Measurements with LiTaO3were consistent.
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Surface Charge DensityLong-Term Measurement of the SCD on LiNbO3
• Surface charge recovers
• Recovering takes half a day [τ = 1.7 x 104 s]
• 0.5%Fe:LiNbO3 shows fast recovery[σ = 10-9 (Ωcm)-1]
• Dry atmosphere did not alter the result
Maxwell relaxation time:
APL 95, 232906 (2009)
Ferroelectric WorkhorseLithium Niobate (LiNbO3)
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"z-cut"-crystal
• Surface charge density: σ = 0.7 C/m²
• Coercive field: Ec = 20 kV/mm
• 180°-domains
Ferr
o-el
ectri
c
d11 = 0d22 = 21d33 = 7
pm/Vd15 = 69d31 = -1P
iezo
-el
ectri
cP
yro-
elec
tric
ΔPs= 80 µC/m2K
Periodically poled LiNbO3 (PPLN)
PPLN (Λ: 8–30µm)Thickness: 500µm
Δn = - ½ n0 r13 E3
r13 = 11.9 pm/VEle
ctro
-op
tic… …
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Piezoresponse Force Microscopy (PFM)Basics
10µm
Ti-indiffusedwaveguide
PPLN
Topography and piezoresponse are readout simultaneously and independently
Lateral resolution ∝ tip radiustypically 20 nm
Sensitivity of PFM: < 0.1pm/V
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Scanning Force Microscopy (SFM) EFM - Electrostatic Force Microscopy
SFM in non-contact mode: tip-sample distance z few nm
Lateral resolution ∝ r and z
Frequency: few 10 kHzAmplitude: few V
Sensitivity: Single(!) charge detection (fN)
SFM in contact mode: tip-sample distance z = 0
Lateral resolution ∝ r
Sensitivity: Piezoresponse of < 0.1pm/V
Frequency: few 10 kHzAmplitude: few V
PFM - Piezoresponse Force Microscopy
PFM versus EFMElectrostatic contribution to PFM?
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• Pyroelectric coefficient:
ΔPs= 80 µC/m2K
(data sheet)
• Surface charge density:
σ = 140 µC/m²
(experimentally determined)
No change of the contrast upon cooling the crystalNo effect of the surface charging on the PFM signal
RSI 84, 043703 (2013)
PFM – Temperature-Dependent MeasurementsDomain evolution in barium titanate (BaTiO3)
20image size 20 x 20 µm² APL 105, 152901 (2014)
PFM – Temperature-Dependent MeasurementsDomain evolution in barium titanate (BaTiO3)
21image size 20 x 20 µm²
Ferroelectrics & Scanning Force Microscopy… using PFM and EFM
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• Visualization of domain patterns
• Evolution of domain patterns across phase transitions
• Determination of the effective surface charge density
Properties of ferroelectric domain wallsWidth? Few nm
And? … they are conductive!
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Domain Wall ConductivityStatus
DWC has first been observed in 2009 in thin film BFO
Since then: exponential increase of research (and publications)
Standard experimental setup: c-AFM
… conductive = less insulating …
D. Meier et al. Nature Materials 11, 284 (2012)
ErMnO3
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Domain Wall Conductivity in Lithium Niobate Temporal evolution over 1.5 days in a 30 µm thin sample
60 x 60 µm²
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Domain Wall Conductivity in Lithium Niobate Quantitative estimates
Dark conductivity of Mg:LiNbO3: σ = 10-15(Ωcm)-1
Conductivity of the 1° inclined domain walls: σ = 0.03 (Ωcm)-1
(assuming a wall width of w = 10 nm)
Conductivity of something real (copper): σ = 109 (Ωcm)-1
A conductive DW is just less insulating than its surroundings …
… but it's non the less fun!
Further investigations …
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From ferroelectric domain patterning …
… towards nanostructuring
+z does not etch−z etches 800nm/h
in 48% HF @RT
Domain Selective Etching
Annealing at T < TcKeeps the ferroelectric domains Smoothens the surface
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Poling of Domains with an SFM TipBasic principle
"doughnut"
"croissant"
"muffin"
JAP 110, 052018 (2011)
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Domains in He-Implanted LiNbO3 SamplesSFM writing & Domain selective etching
1 µm
1µm
Nanotechnology 22 (2011) 285309
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Poling of Domains by UV-LightPoling inhibition
LiNbO3
λ = 275 nmα < 100 nm
T ≈ 1000°C
"poling inhibited domain"
Lithium concentration [mol%]48.5 50.049.549.00
10
20 Stoichiometric
Congruent
EC
[kV
/mm
]
30
20 µm
Fabrication of Waveguides
near-field profile @ 633 nm SEM image of a ridge waveuide
UV-light induced domains & Domain selective etching
APL 97, 151112 (2010)
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Fabrication of Photonic Micro-StructuresUV-light induced domains & Domain selective etching & Annealing
Opt. Express 18, 11508-11513 (2010)
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C UOLUMBIA NIVERSITYIN THE CITY OF NEW YORK
A. Ofan, R. Osgood, Jr. Y. Jing, S. Mailis
Many thanks to the Bonn crew
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Pieter Brueghel the Elder (1530 – 1569)Children's Games 1560
Kun
sthi
stor
isch
es M
useu
m W
ien.